1. Field of the Invention
The present invention relates to an optical low-pass filter and an optical apparatus (image sensing apparatus) using the same and, more particularly, to an optical low-pass filter suitable for a digital still camera, video camera, digital camera, and the like which use a solid-state image sensing element such as a CCD or the like.
2. Related Background Art
Since an image pick-up apparatus such as a digital still camera, video camera, or the like, which uses a two-dimensional solid-state image pick-up element such as a CCD, MOS, or the like, samples an object image at pixel pitches, when an object image having high spatial frequency components is to be sensed, a spurious resolution signal that outputs an aliasing image of high-frequency components as low-frequency components is generated, thus lowering the resolution of an object image. In an image pick-up apparatus which uses a single-plate color solid-state image sensing element as well, upon sensing an object image having high spatial frequency components, a false color signal determined by the layout of a color filter which is set in front of pixels is generated, thus deteriorating the color reproducibility of an object image.
As a conventional means which is inserted in the optical path of an image sensing system to reduce the spurious resolution signal or spurious color signal resulting from high-frequency components of an object image, various optical low-pass filters have been proposed. As the most prevalent one of these optical low-pass filters, an optical low-pass filter which comprises a plane-parallel plate made of a quartz single crystal is known. In general, when the normal to an entrance surface is obliquely set to make a predetermined angle with the optic axis (Z-axis) of a crystal, rays that enter the plane-parallel plate made of quartz as a uniaxial crystal exhibit anisotropy in the Z-axis direction to be separated into ordinary and extraordinary rays, and separated rays parallelly leave the plane-parallel plate. At this time, the separation distance between the ordinary and extraordinary rays is determined by an angle the normal to the entrance surface of the plane-parallel plate makes with the Z-axis of the crystal, and the thickness of the plane-parallel plate.
Optical low-pass filters which utilizes such optical effects of quartz have been proposed in, e.g., Japanese Utility Model Publication Nos. 47-18688 and 47-18689, Japanese Patent Application Laid-Open Nos. 59-75222 and 60-164719, and the like.
Japanese Utility Model Publication Nos. 47-18688 and 47-18689 disclose an arrangement which assumes a stripe-shaped filter as a color filter, and separates rays into ordinary and extraordinary rays using a plane-parallel plate made of, e.g., quartz having birefringence to image them on an image sensing surface so as to reduce a spurious color signal generated when the spatial frequency of an object is synchronized with the color filter. In particular, Utility Model Publication No. 47-18689 discloses an arrangement that cuts out a single crystal of quartz so that its optic axis (Z-axis) makes an angle of around 45° with the entrance and exit surfaces of the plane-parallel plate.
Japanese Patent Application Laid-Open Nos. 59-75222 and 60-164719 disclose an arrangement which assumes a Bayer-matrix filter shown in, e.g., FIG. 5 as a color filter, separates rays into ordinary and extraordinary rays by combining a plurality of birefringence plates to form a plurality of object images which are obtained by separating an object image and are offset by a predetermined pitch, and effectively reduces a spurious resolution signal and spurious color signal generated due to high-frequency components of an object.
An example that uses a single-crystal plate made of a material other than quartz as a birefringence plate is disclosed in Japanese Patent Application Laid-Open Nos. 9-211222, 11-218612, and the like. Japanese Patent Application Laid-Open Nos. 9-211222 and 11-218612 have proposed an optical low-pass filter which uses a plurality of birefringence plates, and forms at least one of these plates using lithium niobate. A single crystal of lithium niobate is a uniaxial crystal like that of quartz but assures a larger difference between the refractive indices of ordinary and extraordinary rays than quartz. For this reason, the thickness of the birefringence plate required for obtaining a predetermined ray separation distance can be decreased.
In the prior art that uses one or a combination of a plurality of birefringence plates of quartz as an optical low-pass filter, since the refractive index difference between ordinary and extraordinary rays is small, the thickness of each birefringence plate must be larger than a given value to assure a predetermined ray separation distance.
In general, when a plane-parallel plate of a uniaxial crystal is prepared so the normal to its entrance surface makes an angle θ with the optic axis (Z-axis), and circularly polarized light perpendicularly enters the plane-parallel plate, an angle φ the traveling direction of ordinary rays makes with that of extraordinary rays in the plane-parallel plate is given by:tanφ=(no2−ne2)sinθcosθ/(ne2cos2θ+no2sin2θ)  (4)where no is the refractive index of ordinary rays, and ne is that of extraordinary rays.
Equation (4) represents the separation distance between ordinary and extraordinary rays per unit thickness of the birefringence plate, and this value is maximum when θ=45°.
If the quartz single crystal has a refractive index no=1.544 of ordinary rays and a refractive index ne=1.533 of extraordinary rays for d-lines, tanφ≅−0.0058 when θ=45°. Hence, when, for example, a solid-state image sensing element has a pixel pitch Ph=10 μm in the long-side direction, and extraordinary rays are offset 10 μm in this direction by a quartz birefringence plate to remove a spurious resolution signal generated in this direction, the quartz birefringence plate must have a thickness d of at least about 1.7 mm. In this manner, when quartz that assures a small refractive index difference between ordinary and extraordinary rays is used as the birefringence plate, the optical low-pass filter becomes thick, thus posing a problem in terms of space factor.
Furthermore, when a plane-parallel plate is inserted in the optical path of a photographing optical system, the following problems are posed in addition to that of space factor.
Rays which are imaged by an ideal lens and pass through the plane-parallel plate are refracted at refraction angles according to the Snell's refraction law which is known to those who are skilled in the art in accordance with incident angles α into the plane-parallel plate, and leave the plane-parallel plate in a direction parallel to the incoming rays. The prospective focal plane of an ideal lens is determined by the paraxial theory that uses an approximation of sinα≅α for the optical length in the plane-parallel plate, and is set at a position offset in the optical axis direction by an amount substituted by an air-converted optical path length d/N which is given using the plate thickness d and refractive index N of the plane-parallel plate. However, when the incident angle of rays that enter the plane-parallel plate increases, and the approximation of sinα≅α deviates from the actual value, rays that emerge from the ideal lens cease to form an image on the prospective imaging surface. More specifically, too much spherical aberration occurs on the optical axis, and astigmatism which becomes too much on the meridional image surface with respect to the sagittal image surface is produced in an off-axis region.
FIGS. 10A to 10C are explanatory views for explaining such phenomenon, i.e., ray aberrations which occur in a birefringence plate when an ideal lens having an exit pupil of F2.0 is set at a position 50 mm separated from an ideal image surface, and a quartz birefringence plate made of a 5-mm thick plane-parallel plate having entrance and exit surfaces perpendicular to the optical axis is set at a position between the ideal lens and an ideal image surface. FIG. 10A illustrates an ideal lens 41 having a focal length=50 mm, a 5-mm thick quartz birefringence plate 42, and a prospective focal plane 43 obtained by a paraxial calculation. Also, 10B and 10C respectively indicate neighbor regions of image surfaces having image heights of 0 mm and 20 mm.
Rays which are imaged by the ideal lens 41 and pass through the plane-parallel plate travel to form an image near the prospective focal plane 43, as shown in FIG. 10A. The neighbor regions 10B and 10C of that image plane are illustrated in an enlarged scale, as shown in FIGS. 10B and 10C. That is, the best image surface position at an image height of 0 mm is located, as indicated by 46 in FIG. 10B, and that at an image height of 20 mm is located, as indicated by 47 in FIG. 10C. As a result, spherical aberration becomes too much at the center of the frame, and astigmatism which becomes too much on the meridional image surface is produced on the periphery of the frame.
In this manner, when the plane-parallel plate is inserted between the photographing optical system and its prospective focal plane, the aforementioned ray aberrations are produced. Hence, it is a common practice to design the photographing optical system which must use a plane-parallel plate such as a quartz birefringence plate in consideration of aberrations produced by the plane-parallel plate. However, an image sensing apparatus with an exchangeable photographing optical system (e.g., a single-lens reflex type digital still camera) which can utilize exchangeable lens systems prepared for silver halide cameras as photographing optical systems cannot take such measure.
In a single crystal of lithium niobate as a uniaxial crystal like quartz, if the refractive index no=2.300 of ordinary rays and the refractive index ne=2.215 of extraordinary rays with respect to d-lines, tanφ≈0.0376 when θ=45°.
Assuming that the solid-state image sensing element has a pixel pitch Ph=10 μm along its long side direction as in the case of quartz, and a lithium niobate birefringence plate shifts extraordinary rays by 10 μm in this direction to remove a spurious resolution signal generated in that direction, since a lithium niobate single crystal has tanφ≅0.0376, the birefringence plate need only have a thickness d of about 0.27 mm. Compared to the aforementioned case using the quartz single crystal, the thickness of the birefringence plate can be reduced to about 0.16 times, and when the lithium niobate birefringence plate is applied to an image sensing system as an optical low-pass filter, a compact image sensing system can be realized while solving the problem of space factor, and the problem of ray aberrations produced by the plane-parallel plate can be alleviated.
Japanese Patent Application Laid-Open Nos. 9-211222 and 11-218612 pay attention to such features of the lithium niobate single crystal.
Assuming that an image sensing apparatus uses a two-dimensional solid state image sensing element which has a pixel pitch=10 μm and an aspect ratio=2:3, and has about 2.5 million effective pixels, the number of pixels in the horizontal direction is around 1,950, that in the vertical direction is around 1,300, and the dimensions of the effective pixels of the solid-sate image sensing element are about 19.5 mm (horizontal direction)×13.0 mm (vertical direction). If an optical low-pass filter made of a plane-parallel plate is relatively closely set in front of such solid-state image sensing element, the plane-parallel plate must have dimensions of at least about 21.0 mm (horizontal direction)×14.5 mm (vertical direction) by adding an effective ray region that considers the solid angle of rays which enter the solid-state image sensing element, and a region for holding the plane-parallel plate itself.
On the other hand, when extraordinary rays are to be separated by the pixel pitch=10 μm in the horizontal direction, if the angle θ the optic axis (Z-axis) of the lithium niobate single crystal makes with the normal to the entrance surface of the plane-parallel plate is set at 45°, the thickness of the plane-parallel plate is about 0.27 mm, as described above. As disclosed in Japanese Utility Model Publication No. 47-18688, when a plane-parallel plate that separates extraordinary rays in a direction which makes an angle of about 45° with the horizontal line of the frame in consideration of removal of the influence of a spurious signal due to frequency components ½ the pixel pitch is used together, the thickness of this plane-parallel plate is around 0.19 mm. Since the thicknesses of these plane-parallel plates are substantially 1% or less of the diagonal length determined by the aforementioned outer dimensions, these plates easily crack due to insufficient mechanical strength.
Japanese Patent Application Laid-Open Nos. 9-211222 and 11-218612 mentioned above proposed arrangements of optical low-pass filters in which lithium niobate and quartz are adhered. These prior arts can reduce the thickness of the optical low-pass filter compared to that which is made of quartz alone, but the aforementioned problems of space factor and optical performance of the photographing optical system remain unsolved.
A low-pass filter of the present invention assumes use in, e.g., a digital camera, video camera, and the like, and is especially suitable for a single-lens reflex type digital camera which can effectively utilize various exchangeable lenses prepared as those to be mounted on a single-lens reflex camera that uses a silver halide film. For this purpose, when the low-pass filter is designed to have a sufficiently low profile for use in the single-lens reflex type digital camera main body, it must prevent excessive drop of the resolving powers of the exchangeable lenses and must not cause any ghost or flare when it is used in an image sensing system.
Each exchangeable lens for a single-lens reflex camera that uses a silver halide film is designed to assure spaces for a pivotal mirror and a focal plane shutter so as to realize a TTL finder free from any parallax by maintaining a sufficiently large distance (back focus) from the lens surface on the most image side to the focal plane.
In order to realize a single-lens reflex type digital camera that uses a solid-state image sensing element in place of a silver halide film, the image sensing surface of the solid-state image sensing element can be set at a position equivalent to that of the silver halide film. However, as disclosed in the first prior art, when an optical low-pass filter that uses a plurality of quartz birefringence plates is arranged in the camera, it unwantedly interferes with the pivotal mirror and focal plane shutter.
On the other hand, an exchangeable lens for a single-lens reflex camera that uses a silver halide film normally does not assume the presence of any reflecting member between the lens surface on the most image side and the focal plane. That is, when an optical member having a predetermined reflectance or higher with respect to the visible wavelength range is inserted between the photographing optical system and its focal panel, it tends to produce harmful rays such as surface-reflected ghost, flare spot, and the like that decrease the contrast of an object image. Hence, the reflectance of the optical low-pass filter in the visible wavelength range must be sufficiently low.